Header

    pub const Header = struct {
        block_type: BlockType,
        size_format: u2,
        regenerated_size: u20,
        compressed_size: ?u18,

        /// Decode a literals section header.
        pub fn decode(in: *Reader, remaining: *Limit) !Header {
            remaining.* = remaining.subtract(1) orelse return error.EndOfStream;
            const byte0 = try in.takeByte();
            const block_type: BlockType = @fromBackingInt(@intCast(byte0 & 0b11));
            const size_format: u2 = @intCast((byte0 & 0b1100) >> 2);
            var regenerated_size: u20 = undefined;
            var compressed_size: ?u18 = null;
            switch (block_type) {
                .raw, .rle => {
                    switch (size_format) {
                        0, 2 => {
                            regenerated_size = byte0 >> 3;
                        },
                        1 => {
                            remaining.* = remaining.subtract(1) orelse return error.EndOfStream;
                            regenerated_size = (byte0 >> 4) + (@as(u20, try in.takeByte()) << 4);
                        },
                        3 => {
                            remaining.* = remaining.subtract(2) orelse return error.EndOfStream;
                            regenerated_size = (byte0 >> 4) +
                                (@as(u20, try in.takeByte()) << 4) +
                                (@as(u20, try in.takeByte()) << 12);
                        },
                    }
                },
                .compressed, .treeless => {
                    remaining.* = remaining.subtract(2) orelse return error.EndOfStream;
                    const byte1 = try in.takeByte();
                    const byte2 = try in.takeByte();
                    switch (size_format) {
                        0, 1 => {
                            regenerated_size = (byte0 >> 4) + ((@as(u20, byte1) & 0b00111111) << 4);
                            compressed_size = ((byte1 & 0b11000000) >> 6) + (@as(u18, byte2) << 2);
                        },
                        2 => {
                            remaining.* = remaining.subtract(1) orelse return error.EndOfStream;
                            const byte3 = try in.takeByte();
                            regenerated_size = (byte0 >> 4) + (@as(u20, byte1) << 4) + ((@as(u20, byte2) & 0b00000011) << 12);
                            compressed_size = ((byte2 & 0b11111100) >> 2) + (@as(u18, byte3) << 6);
                        },
                        3 => {
                            remaining.* = remaining.subtract(2) orelse return error.EndOfStream;
                            const byte3 = try in.takeByte();
                            const byte4 = try in.takeByte();
                            regenerated_size = (byte0 >> 4) + (@as(u20, byte1) << 4) + ((@as(u20, byte2) & 0b00111111) << 12);
                            compressed_size = ((byte2 & 0b11000000) >> 6) + (@as(u18, byte3) << 2) + (@as(u18, byte4) << 10);
                        },
                    }
                },
            }
            return .{
                .block_type = block_type,
                .size_format = size_format,
                .regenerated_size = regenerated_size,
                .compressed_size = compressed_size,
            };
        }
    }